1. Field of the Invention
The present invention relates to a method and an apparatus for heat treating a semiconductor wafer or a glass substrate.
2. Description of the Related Art
Heretofore, in order to form a diffused layer, a silicon oxide film or a silicon nitride film on a semiconductor wafer or a glass substrate, various types of heat treating apparatuses are employed. In general, in these apparatuses, a substrate to be heat treated is heat treated by heating the substrate to be heat treated, contained in a reaction vessel (i.e., a heating chamber) and introducing "inert gas" or "reactive gas" or "reactive gas" into the vessel.
The following conditions are required as performances of the heat treating apparatus:
(1) The resistance and the depth of a diffused layer to be formed and the thicknesses of a silicon oxide film, a silicon nitride film are uniform on the surfaces of wafers and between the wafers, and its reproducibility is high.
(2) No "slip" or "warpage" occurs in the wafer due to "thermal distortion" caused by a temperature difference.
(3) A large quantity of wafers can be treated per one operation.
(4) Treating time per one operation is short.
(5) When substrates to be treated are conveyed in or out of the vessel, no air is simultaneously introduced into the substrate, and no oxide film is grown on the surface of the substrate to be treated due to the air.
Heretofore, a configuration in which a heat insulator is mounted at the periphery of a heater or a linear tube made of silicon carbide is provided between a heater and a process tube has been frequently employed so as to enhance the uniformity of temperatures in the steps of oxidizing and heat treating at high temperatures near 1,000.degree. C. in a semiconductor integrated circuit device. When a semiconductor wafer is heat treated at high temperatures by using the above-described semiconductor heat treating apparatus, it frequently occurs to mix the air at the same time in the case of conveying the semiconductor wafer into or out of a vessel. Therefore, an oxide film is formed in an atmosphere which is not controlled. As a result, the uniformity of a film or a diffused layer formed by the heat treatment at high temperatures might be deteriorated or the quality of the film might be lowered. In order to prevent the malfunction of the above-described treatment, the following method is employed. That is, a method having the steps of lowering temperatures at the time of inserting a semiconductor wafer into a vessel, extracting the mixed air with gas having high purity such as nitrogen, argon or oxygen, then heating it to high temperatures, lowering the temperature after the heat treatment, and then removing the semiconductor wafer is employed.
However, when a heat treating apparatus having a heat source of large heat capacity is employed for the above-described method, it takes a plenty of time to oxidize at a desired temperature and to rise and fall the temperatures before and after the steps except the heat treatment, resulting in a decrease in productivity or disabling the depth of an impurity diffused layer to be held shallow. If the semiconductor wafer is abruptly removed from the vessel to be cooled after it is oxidized or heat treated, a large temperature difference occurs on the surface of the semiconductor wafer, and a slip occurs or a warpage occurs to be deformed.
As a method of solving the above-described problems, a heat treating apparatus in which a heater is wound on the outer periphery of a quartz process tube, the outer periphery of the process tube is covered with a high reflection film to reduce the heat capacity of a heat source, and "temperature rising and falling velocities" are high is proposed. However, if the maximum output of the heater is intended to be improved to the degree capable of heat treating at high temperatures in the apparatus of this configuration, its heat capacity is increased, and its temperature falling velocity is lowered. In comparison with the conventional heat treating apparatus in which the temperature rising and falling velocities are about 10.degree. C. per min., about 2.degree. to 3.degree. C. per min., respectively, the temperature rising and falling velocities of the improved apparatus are 30.degree. C. per min., of relatively high performance are obtained, but this value is not still sufficiently high.
As the other method, a method having the steps of contacting a substrate to be treated with "a heater" made of carbon installed in a vessel and rising its temperature by means of induction heating has been known. However, according to this method, since the substrate to be treated is contacted with the heater, the volume of the vessel is increased. As a result, it was disadvantageous to treat the substrate to be treated of a large diameter. Further, a method disclosed in Unexamined Published Japanese Patent Application No. 61-191015, i.e., a method having the steps of mounting a heater in a vessel, disposing semiconductor wafers at a predetermined interval horizontally in the heater, and heating the wafers by a heat source provided out of the vessel has been proposed. However, according to this method, if the distance between the heat source and the substrate to be treated is near and the diameter of the substrate to be treated is large, there arises a problem in which its temperature distribution occurs in the radial direction of the substrate to be treated.
In principle, the substrate to be treated by heat in such a manner that heat rays radiated from a heater are absorbed by the substrate to be treated. However, if a plurality of substrates to be treated are mounted perpendicularly to the heater, when the heating velocity of the heater is set to a large value so as to accelerate the temperature rising velocity, the plurality of substrates to be heated mounted in parallel with each other form shades to each other. Therefore, the temperature change of the periphery of the substrate to be treated becomes faster than that of the center of the substrate at the temperature rising and falling times. As a result, it was difficult to make the radial temperature distribution of the substrate to be treated uniform.
As described above, in order to heat treat a large number of wafers at once (in a batch type), it is desirable to mount a plurality of wafers in parallel with each other perpendicularly to a heater in a reaction vessel. However, according to the conventional method, when the wafers are "abruptly heated or cooled" and further "unloaded", a temperature difference occurs in the radial direction of the wafer, causing a slip or a crystal defect to occur due to thermal distortion or the wafer to be warped. If the wafer of large diameter such as 8 inches or larger is treated, a large temperature distribution difference occurs on the surface of the wafer (particularly in the radial direction). Therefore, it is difficult to further accelerate "the temperature rising velocity" and "the temperature falling velocity" due to causes of malfunctions.